Method of depositing diamond and diamond light emitting device

ABSTRACT

A method of depositing high quality diamond films and a light emitting device are described. The deposition is carried out in a reaction chamber. After disposing a substrate to be coated in the chamber, a carbon compound gas including a C--OH bond is introduced together with hydrogen thereinto. Then, deposition of diamond takes place in a magnetic field by inputting microwave energy. The present invention is particularly characterized in that the volume ratio of the carbon compound to hydrogen introduced into the reaction chamber is 0.4 to 2; the pressure in said reaction chamber is 0.01 to 3 Torr; the temperature of the substrate is kept between 200° to 1000° C. during deposition; and the input energy of the microwave is no lower than 2 KW. By this method, uniform and high quality diamond films can be formed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of depositing diamond films having uniform thickness by means of microwave enhanced CVD and a light emitting device formed by this method.

2. Description of the Prior Art

Many attempts have been proposed to deposit diamond films by microwave assisted CVD, since diamond is regarded promissing as a future material for electronic devices. Particularly, diamond light emitting devices have attracted interest of many researchers, because of its high thermal resistance, chemically stabilities and low price suitable for mass production, in view of a great demand for light emitting devices in the market. An example of such devices is described in Japanese Patent Application No. sho 56-146930. It is, however, very difficult to deposit high quality diamond films having uniform thicknesses at high deposition speeds required for production on a commercial base.

The deposition conditions of prior art diamond formation have been such that; the reactive gas is composed of a hydrocarbon diluted by hydrogen at about 1-5%; the pressure of the reactive gas during deposition is about 5-20 Torr; the input power of microwaves is 1-5 KW; and the deposition is carried out in a magnetic field of 2 KG. Crystal surfaces can be observed by a microscope even in the surface of diamond films deposited by such a prior art method. The deposition speed achieved by these deposition methods, however, have been limited up to 667 angstrom thickness per hour, i.e. it takes 15 hours to deposit a one micrometer thick diamond film. Furthermore, the area coated by diamond is limited only to 3 cmφ keeping the disparity in thickness within 10%. Still further, the quality of diamond deposited is not so high. Graphite is also deposited and gets mixed into diamond films. The graphite component has been confirmed by Raman spectroscopic anylysis.

It is also known to use carbon monoxide diluted by hydrogen at 1-5%. The pressure in a reaction chamber in this case is 0.1-1 Torr. The deposition speed, however, is also low. It takes more then 10 hours to deposit a diamond film of one micrometer thickness even when input energy is applied at 3-5 KW. The deposited area within which a uniform diamond film can be deposited is only up to 5 cm square. The more problematic disadvantage of this method is etching of inner walls of the chamber. The etching causes metallic contamination in diamond films deposited. The metal is such as Fe constituting the wall. This contamination was confirmed by Auger electron spectroscopy. The etching product is coated also on a window through which microwaves are admitted to the chamber and form an impediment of the microwave introduction.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of depositing diamond of high quality at a high speed.

It is another object of the present invention to provide a diamond light emitting device having excellent emission performance.

Additional objects, advantages and novel features of the present invention will be set forth in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the present invention. The object and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.

To achieve the foregoing and other object, and in accordance with the present invention, as embodied and broadly described herein, microwave-assisted chemical vapor reaction is carried out using a carbon compound gas which includes a C--OH bond. The carbon compound is introduced into a deposition chamber together with hydrogen at a volume ratio of the carbon compound gas/hydrogen =0.4 to 2.0. The deposition takes place in a magnetic field when microwaves energy is inputted to the reactive gas at no lower than 2 KW. The pressure of the reactive gas is kept between 0.01 to 3 Torr during the deposition. The substrate temperature is kept between 200° to 1000° C.

The most effective low speed of the reactive gas passing through the depositon space is 30 to 600 cm/sec preferably to 35 to 200 cm/sec. The features of the present invention include the relatively high flow speed, the relatively low pressure and the relatively high proportion of the carbon compound gas to hydrogen as compared with conventional methods. By this method, diamond films consisting of sp³ bonds can be deposited witin 10% disparity in thickness at no lower than 0.5 micrometer/hour, typically at no lower than 1 micrometer/hour.

In accordance with the present invention, oxygen atoms are automatically introduced into the diamond films. The oxygen atoms function as recombination centers and when applied for a light emitting device the emission efficiency is improved. The density of oxygen in the deposited diamond is for 1×10¹⁸ to 6×10²⁰ cm³. The diamond films can be doped with impurities such as P or B in order to make impurity semiconductor films of n- or p-types. For n-tipe semiconductors, a dopant of VI Group can be also used. The VI Group element makes the semiconductors to be of n-type by causing lattice defects rather than by functioning as a donnor.

It is very important for the present invention to use a carbon compound gas having a C--OH bond. When CO or CO₂ is used together with such as CH₄ as conventional methods, diamond films containing oxygen atoms can be depositied. The films are, however, necessarily contaminated by iron ions which are brought from etching action of the inner wall of the reaction chamber. In this conventional case, oxygen ions are produced as lone atoms which excert strong etching action upon the wall. The inventor assumes that in case of the present invention OH or H₂ O radicals are produced instead of lone O atoms. The etching effect of such OH or H₂ O radicals is not so strong to remove iron from the inner wall but sufficient to remove graphite from the deposted diamond.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated in and form a part of the invention and, together with the description, serve to explain the principles of the invention.

FIG. 1 is a cross sectional view of a chemical vapor reaction apparatus for depositing diamond in accordance with the present invention.

FIG. 2 is a graphic diagram showing a result of Raman spectroscopic analysis of a diamond film deposited in accordance with the present invention.

FIGS. 3 and 4 are photographs showing the surface conditions of diamond films deposited in accordance with the present invention.

FIG. 5 is a graphic diagram showing XRD spectra of a diamond film deposited in accordance with the present invention.

FIGS. 6(A) to 6(C) are cross sectional views showing a method of manufacturing diamond light emitting devices in accordnace with the present invention.

FIGS. 7(A) to 7(C) are cross sectional views showing a modification of the method of manufacturing a light emitting device illustrated in FIGS. 6(A) to 6(C).

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, a microwave-assisted CVD apparatus provided with associated Helmholtz coils 17 and 17' for use in depositing diamond films in accordance with the present invention is shown. The apparatus comprises a vacuum chamber defining a reaction (deposition) space 19 therein, a microwave generator 18 connected to the chamber through an attenuater 16 and a quartz window 15, a gas introduction system having four inlet ports 21 to 24, a gas evacuation system having a wide-range turbo molecular pump 26 and a rotary pump 27 for roughing coupled with the chamber through a pressure controlling valve 25 and a substrate holder 13 provided in the chamber and with a substrate position adjusting mechanism 12 for supporting a substrate 1 at an appropriate position. By the use of the adjusting mechanism 12, the axial position of the holder can be adjusted in order to change the volume of the reactive spece 19. The evacuation system functions both as a pressure controller and as a stop valve. The pressure in the chamber is adjusted by means of the valve 25. The inside of the chamber and the holder 13 are circular and coaxial with each other. The diameter of the circular inside of the chamber is 170 mm. The diameter 28-1 of the circular holder 13 is chosen to be 140 mm so that the clearance between the chamber wall and the holder 13 is about 15 mm. The thickness 28-2 of the holder 1 is 10 to 50 mm. The clearance may be determined between 3 and 30 mm in general. By this configuration, the reaction space 19 is confined in order not to substantially loose input energy and the reflection of microwaves inputted is only about 5%.

Next, a method of depositing diamond films by means of the apparatus will be described in accordance with the invention. The substrate is, for example, a single crystalline silicon semiconductor wafer of 2 to 6 inches, e.g. 4 inches diameter. The surface of the substrate is preferably given scraches which form focuses for crystalline growth. The scraches are formed for example by putting the substrate in a liquid in which diamond fine particles are dispersed and applying ultrasonic waves thereto for 1 minute to 1 hour, or by bombarding the surface with diamond powders. After fixing the substrate 1 on the holder 13 with a keeper 14, the pressure in the reaction space 19 is reduced to 10⁻⁴ to 10-¹⁷ Torr by means of the evacuation system followed by introduction of a reactive gas to a pressure of 0.01 to 3 Torr, typically 0.1 to 1 Torr, e.g. 0.26 Torr. The reactive gas comprises an alcohol such as methyl alcohol (CH₃ OH) or ethyl alcohol (C₂ H₅ OH) diluted with hydrogen at a volume ratio of alcohol/hydrogen=0.4 to 2. The hydrogen is introduced throught the port 21 at 100 SCCM and the alcohol through the port 22 at 70 SCCM for example. The coils are energized during the deposition to induce a magnetic field having a maximum strength of 2 K Gauss and a resona-ting strength of 875 Gauss at the surface of the substrate 1 to be coated. Then, microwaves are applied at 1 to 5 GHz, e.g. 2.45 GHz in the direction parallel to the direction of the magnetic field to cause ionized particles of the reactive gas in the form of plasma to resonate therewith in the magnetic field. As a result, a polycrystal-line film of diamond grows on the substrate. 2 hour deposition for example can form a diamond film of 0.5 to 5 micrometer thickness, e.g. 1.3 micrometers thickness. During the deposition of diamond film, carbon graphite is also deposited. However, the graphite, which is relatively chemically unstable as compared with diamond, react with radicals which also occur in the plasma of the alcohol and is removed form the diposited film. The temperature of the substrate 1 is elevated to 200° C. to 1000° C., typically 300° C. to 900° C., e.g. 800° C. by microwaves. If the substrate temperature is too elevated, water cooling is effected to the substrate holder 13.

The low speed of the reactive gas 20 in the reaction space 19 is regulated by controlling the valve 25. By virtue of the molecular pump 26, the speed can be adjusted from 15 cm/sec up to 1000 cm/sec even at 0.01 Torr in the reaction space 19. From the view point of the deposition performance, speeds of 30 to 600 cm/sec typically 35 to 200 cm/sec are appropriate.

In accordance with experiments, crystalline surfaces of single carbon crystals were clearly observed partially in the films even deposited at a substrate temperature of 200° C. When deposited at a substrate temperature of 300° C., single crystalline surfaces were observed throughout the films deposited. FIG. 2 is a diagram showing Raman spectroscopic analysis of a diamond film deposited in accordance with the present invention. The sharp peak appearing at 1336 cm⁻¹ indicates existence of diamond. There is hardly found a peak at 1490 cm⁻¹ indicative of existence of graphite. It has been confirmed by Auger elecron spectrometry that few iron or stainless contamination occurs in deposited films. It has also been confirmed by secondary ion mass spectroscopy that the diamond films include oxygen atoms at 1×10¹⁸ to 6×10²⁰ cm⁻³. The oxygen concentration depended upon the pressure of the reactive gas and the input energy of microwaves during deposition.

Depending upon the purposes, some impurities may be introduced into diamond films during deposition. Examples of such impurities include B, S, Se and Te. In case of B, B(CH₃)₃ may be introduced as a dopant together with the reactive gas at a volume ratio of dopant gas/alcohol=0.005 to 0.03. In case of S, H₂ S or (CH₃)₂ S may be introduced as a dopant together with the reactive gas at a volume ratio of dopant gas/alcohol=0.001 to 0.03. In the same manner, H₂ Se, H₂ Te, (CH₃)₂ Se and (CH₃)₂ Te can be used as dopant gases. Also, elements of Group IIb such as Zn and Cd can be introduced using a dopant gas of hydrogen or organic compound thereof, or elements of Group Vb can be introduced using NH₃, PH₃ or AsH₃ for example.

The above deposition process was repeated at diverse reaction pressures and with reactive gases diluted with hydrogen at divese ratios. The alcohol used was methanol. The input power of microwaves was 2 KW. The appearences of crystalline conditions of diamond films deposited were examined by a scanning electron microscope and evaluated into three ranks as shown on the most right column of Table 1 with correspondence to the volume ratio of CH₃ OH/H₂, the substrate temperature and the pressure of the reactive gas (CH₃ OH/H₂). In the table, mark ⊚ indicates that squares and/or triangles of crystal surfaces were observed throughout the diamond surface; mark indicates that the diamond surface was covered partially by crystal surfaces; and X indicates that no crystal surface was observed. FIG. 3 is a photograph showing the crystalline surface of No. 1 sample for reference.

                  TABLE 1                                                          ______________________________________                                               volume ratio                                                                               Temperature                                                                               Pressure                                                                              Condition                                  No.   of CH.sub.3 OH/H.sub.2                                                                     of substrate                                                                              of the gas                                                                            of the film                                ______________________________________                                         1     0.7         800° C.                                                                            0.2 Torr                                                                              ⊚                           2     0.8         600° C.                                                                            0.1 Torr                                                                              ⊚                           3     0.7         200° C.                                                                            0.2 Torr                                          4     0.7         400° C.                                                                            0.03 Torr                                         5     0.4         800° C.                                                                            0.15 Torr                                                                             ⊚                           6     0.4         800° C.                                                                            3.0 Torr                                          7     2.0         800° C.                                                                            0.27 Torr                                         8     0.6         800° C.                                                                            5.0 Torr                                                                              X                                          9     0.6         800° C.                                                                            0.01 Torr                                                                             X                                          ______________________________________                                    

                  TABLE 2                                                          ______________________________________                                                                  CH.sub.3 /H.sub.2                                                                      Total                                         No.   Pressure H (l/min) (l/min) (l/min)                                                                               F. Speed                               ______________________________________                                         1     0.03     2533      1773    4266   313                                    2     0.1      760       532     1292   95                                     3     0.26     292       204     496    36.4                                   4     0.6      126       88.2    214    15.7                                   5     1.0      76        53.2    129    9.5                                    6     0.03     2533      2533    5066   372                                    7     0.1      760       760     1520   111.8                                  8     0.26     292       292     584    42.9                                   9     0.6      126       126     252    18.5                                   10    1.0      76        76      152    11.2                                   11    0.03     2533      5066    7599   558                                    12    0.1      760       1520    2280   167.6                                  13    0.26     292       584     876    64.4                                   14    0.6      126       252     378    27.4                                   15    1.0      76        152     228    16.8                                   16    0.03     2533      1266.5  3799.5 279                                    17    0.1      760       380     1140   83.8                                   18    0.26     292       146     438    32.2                                   19    0.6      126       63      189    13.9                                   20    1.0      76        38      114    8.4                                    ______________________________________                                    

Next, the deposition procedure was repeated with diverse flow speeds of the reactive gas through the reaction space. The inner area of the reaction chamber was 2207 cm². The input energy of microwaves was 2 KW to 8 KW. The substrate temperature was 800° C. Table 2 shows the pressures (Torr) in the reaction space, the introduction rates (liter per minute) of hydrogen and methane, the total introduction rates thereof and the flow speeds (centimeter per second) of the reactive gas. Table 3 shows the deposition speeds (D.S.), the crystalline conditions (C.C.), the existence of graphite (E.G.). The existence of graphite was confirmed by Raman spectroscopic analysis. In the cases of Nos. 1 to 5, hydrogen and methane were supplied under atmospheric pressure respectively at 100 SCCM and 70 SCCM (Nos. 1 to 5), at 100 SCCM and 100 SCCM (Nos. 6 to 10), at 100 SCCM and 200 SCCM (Nos. 11 to 15) and at 100 SCCM and 50 SCCM (Nos. 16 to 20). The microscopic photograph of Sample No. 3 (4 KW) is shown in FIG. 4 for reference. The XRD (X-ray diffraction) spectra of Sample No. 3 is shown in FIG. 5. As seen from the spectra, count peaks appeared corresponding to crystalline planes of diamond.

                  TABLE 3                                                          ______________________________________                                         No.    D.S.    C.C.    E.G.   D.S.  C.C.  G.                                   ______________________________________                                         (Input Energy = 2KW)                                                                               (Input Energy = 4KW)                                       1      <0.1    --      --     0.25  ⊚                                                                     no                                   2      0.93    ⊚                                                                       no     0.45  ⊚                                                                     no                                   3      0.4     ⊚                                                                       no     0.65  ⊚                                                                     no                                   4      0.48            no     0.85        no                                   5      0.65    X       yes    --    X     yes                                  6      0.8     ⊚                                                                       no     1.5   ⊚                                                                     no                                   7      1.0     ⊚                                                                       no     1.7   ⊚                                                                     no                                   8      1.7     ⊚                                                                       no     2.3         no                                   9      --              yes    --    X     yes                                  10                                                                             11     1.9     ⊚                                                                       no     2.5   ⊚                                                                     no                                   12     2.7     ⊚                                                                       no     3.2   ⊚                                                                     no                                   13     3.5             no     4.5         no                                   14     --              yes    5.3   X     yes                                  15                                  X                                          16     <0.1    --      --     0.15  ⊚                                                                     no                                   17     0.2     ⊚                                                                       no     0.3   ⊚                                                                     no                                   18     0.35    ⊚                                                                       no     0.5   ⊚                                                                     no                                   19     0.58            no     0.7         yes                                  20     0.9             no     0.95  X     yes                                  ______________________________________                                    

Referring now to FIG. 6, a method of forming a light emitting device utilizing the present invention will be explained. A diamond coating 2 is deposited on a p-type silicon semiconductor substrate 1 of a 4-inch wafer to a thickness of 0.5 to 3.0 micrometers, e.g. 1.3 micrometers, by the microwave-assisted plasma CVD method in a magnetic field as described above. During the deposition, B(CH₃)₃ is introduced as dopant gas together with CH₃ OH diluted by hydrogen at a volume ratio of CH₃ OH/H₂ =0.8. The volume ratio of B(CH₃)₃ /CH₃ OH is 0.03. The oxygen concentration of the diamond film is 1×10¹⁸ to 6×10²⁰ cm⁻³. The diamond film is then given scratches 10 by bombarding its surface with diamond powders. The scratches 10 form focuses for growing diamond crystals therefrom. On the scratched surface, diamond deposition is carried out again to form a second diamond film 3 with a thickness of 0.5 micrometer. The reactive gas in this depostion consists of H.sub. 2, CH₃ OH and (CH₃)₂ S introduced at volume ratios of (CH₃)₂ S/CH₃ OH=0.03 and CH₃ OH/H₂ =0.7. Then, an upper transparent electrode 4 made of indium tin oxide (ITO) and to lead electrode 5 are deposited by vapor evaporation or sputtering. The lead electrode 5 may be a multilayered metallic film consisting of, e.g. aluminum, silver, chromium and/or molybdenum films. The upper electrode 4 is severed by laser scribing into a number of 0.6 mm×0.6 mm segments on each of which the lead electrode 5 of 0.15 mm diameter is formed. The width of the scribing lines is 100 micrometers. Accordingly, 5000 light emitting elements of 1 mm×1 mm are formed within the 4-inch wafer except for a margin of 4 mm.

By this structure, light emission can be realized by causing electrical current to pass through the the diamond crystals of the film. The current induces recombination of electron-hole pairs between mid-gap states (radiation centers), between the mid-gap states and a valence band, between a conduction band and the mid-gap states and between the conduction band and the valence band. Oxygen atoms automatically introduced function as radiation centers. The spectrum of light emitted from a diamond film is determined by differential energy levels between the mid-gap states, the bottom of the conduction band and the apex of the valence band. Depending upon the differential levels, it is possible to emit blue of green light or radiation having continuous spectrum of wavelengths over a relatively wide range such as white light.

When a voltage of 10 to 100 V (e.g. 60 V) was applied across the diamond film 5 of a diamond light emitting device formed in accordance with the above procedure by means of the upper electrode 4 and the substrate 1 functioning as a counterpart lower electrode, diamond emitted green visual light at 6 cd/m² by virtue of current passing therethrough. The voltage may be applied as a DC voltage or as a pulse train at no higher than 100 Hz of a duty ratio of 50%. When products capable of emitting light at no lower than 5 cd/m² were regarded as acceptable, the yield of no lower than 85% was obtained.

Referring now to FIG. 7, a modification of the example shown in FIG. 7 will be described. A boron-doped diamond coating 2 is deposited on a p-type silicon semiconductor substrate 1 of a 4-inch wafer to a thickness of 1.3 micrometers in the same manner as described above. An element of Group Vb is introduced to the surface of the diamond film 2 to form a doped surface 30 by ion implantation. On the doped surface 30, diamond deposition is carried out again to form a second diamond or silicon carbide film 31 of an n-type with a thickness of 0.5 micrometer. The reactive gas in this depostion consists of H₂, CH₃ OH, SiH₄ and PH₃ introduced at volume ratios of SiH₄ /CH₃ OH=PH₃ /CH₃ OH=0.03 and CH₃ OH/H₂ =0.7. An n-type silicon semiconductor film may be deposited in place of the silicon carbide film 31 by omitting the use of the alcohol. Then, an upper transparent electrode 5-1 made of a thin chromium film and a lead electrode 5-2 are deposited by vapor evaporation or sputtering. The lead electrode 5 may be a multilayered metallic film consisting of, e.g. alminum. The upper electrode 4 is sectioned into a number of 0.6 mm×0.6 mm effective regions (region (b) in the figure) by etching with a suitable mask of photoresist. Within each effective region, the upper electrode 4 is patterned to form a plurality of strips. At the same time, the second film 31 is etched with the same mask. The lead electrode 5 of 0.1 mm×0.1 mm is formed in each of effective region. Accordingly, nearly 10000 light emitting elements of 0.8 mm×0.8 mm (region (a) in the figure) are formed within the 4-inch wafer except for a margin of 4 mm. When a 40 V was applied between the electrode 5-1 and the substrate 1, 20 cd/cm² light emission was obserbed.

The foregoing description of preferred embodiments has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form described, and obviously many modifications and variations are possible in light of the above teaching. The embodiment was chosen in order to explain most clearly the principles of the invention and its practical application thereby to enable others in the art to utilize most effectively the invention in various embodiments and with various modificartinos as are suited to the particular use contemplated. Although in the above examples alcohols are used as the reactive gas of carbon compound, other carbon compounds having a C--OH bond can be used. Examples include compounds having benzen rings such as C₆ (OH)₆, C₆ H₃ (OH)₃, etc, or mixtures of compounds having a C--OH bond and another carbon compound having a C═C bond such as C₂ H₂ mixed with CH₃ OH. These carbon compounds are used by diluting with hydrogen. The deposition speed can be improved by a factor of 2 to 5 by the use of these alternatives. The cost of the reactive gas, however, becomes more expensive in these cases.

The present invention is broadly applicable to any electric device comprising a diamond film. These electric devices can be formed on a single substrate, i.e. an integrated circuit device which may consists of diamond light emitting devices, diamond diodes, diamond transistors, diamond resistances, diamond capacitors and the like. Of course, it is possible to severe a single substrate, after a number of diamond devices are formed on the substrate, into indivisual separate devices. 

What is claimed is:
 1. A method of depositing diamond film comprising:disposing a substrate into a reaction chamber; introducing a carbon compound gas having at least one hydroxyl group together with hydrogen into said reaction chamber; inputting electromagnetic energy to said reaction chamber for activating said carbon compound gas: and depositing diamond on said substrate; wherein the volume ratio of said carbon compound is hydrogen introduced into said reaction chamber is 0.4 to 2, and the temperature of said substrate is kept between 300° to 900° C. during deposition.
 2. The method of claim 1 wherein said electromagnetic energy is a microwave energy and a magnetic field is further induced in the reaction chamber.
 3. The method of claim 1 wherein the pressure is 0.01 to 3 Torr.
 4. The method of claim 1 wherein the inside of said reaction chamber is evacuated to 10⁻⁴ to 10⁻⁷ Torr in advance of said introducing step.
 5. The method of claim 1 wherein a dopant gas is introduced into said chamber together with said carbon compound gas and hydrogen.
 6. The method of claim 5 wherein said dopant gas is a boron compound gas.
 7. A method of manufacturing an electronic device comprising the steps of:depositing diamond on a p-type semiconductor substrate by plasma CVD; forming a n-type semiconductor layer made of diamond, silicon carbide or silicon; and forming an electrode arrangement on said semiconductor.
 8. The method of claim 7 wherein said diamond contains oxygen.
 9. The method of claim 8 wherein the concentration of said oxygen is in the range of 1×10¹⁸ to 6×10²⁰ atoms/cm³.
 10. The method of claim 7 wherein said diamond is added with boron.
 11. The method of claim 7 wherein said device is a diamond light emitting device. 